Soil humic acid decreases biological uranium(VI) reduction by Shewanella putrefaciens CN32

Biological reduction of uranium(VI) by Shewanella putrefaciens CN32 was investigated in the presence of soil humic acid (SHA). Experiments were performed under resting cell conditions with uranyl acetate as the electron acceptor and sodium lactate as the electron donor in a NaHCO₃ or PIPES/NaHCO₃ buffer. SHA significantly decreased the final extent of U(VI) bioreduction (100% uranium bioreduced without humics vs. 50% uranium bioreduced with humics). Dissolved (0.2-μm filter) U(VI) was measured by kinetic phosphorescence analysis where samples were kept anoxic during one measurement, and split samples were oxidized and remeasured. The difference between the anoxic and oxidized measurements was operationally defined as "U(IV) < 0.2-μm." U(IV) <0.2-μm was detected only in samples containing SHA suggesting that soluble U(IV)-SHA complexes were formed or SHA facilitated the transport of U^IVO₂(s) nanoparticles through the filter. Identical experiments conducted with nitrate instead of U(VI) showed that SHA interrupted electron transport to nitrate, and may interact similarly with U(VI). These results suggest that humic materials may influence U immobilization by decreasing U(VI) reduction and enhancing the solubility of U(IV).